Corundum linings in the high-temperature and transition zones

Corundum (α-Al₂O₃) has a high melting point and good high-temperature strength, a lower thermal expansion than many basic bricks, and a relatively low thermal conductivity. Placing a corundum working lining in the high-temperature zone and in the upper and lower transition zones is a way to keep the hot face intact where the coating is unstable and the kiln is often stopped and started.

Basic products — magnesia-iron spinel and magnesia-alumina spinel — have their place in the burning and transition zones, but the shell temperature can run high, and the hot face is prone to end-spalling when the kiln is stopped and started. The thermal-shock resistance, flexibility and lower conductivity of a corundum brick are aimed at that weakness.

In silica-mullite bricks in the transition zone, grate cooler and wind-guard wall, silicon carbide can oxidise at high temperature; once the glaze is lost, the brick is liable to crack. A corundum working lining does not follow that oxidation path, and it is also more wear-resistant, which is why it often appears in these hot, erosive positions. It does not replace basic brick throughout the kiln: in the burning zone it is more often laid together with magnesia-iron spinel, so that coating adhesion and thermal-shock resistance are shared.